Due to its high paramagnetic behavior it is challenging for chemists to develop fluorogenic receptors specific for Cr (III) detection with fluorescence on response, as it is well known to quench the luminescence of a fluorophore. Further, due to the high solvation enthalpy of Cr (III)-ion in aqueous medium, it is also difficult to find an appropriate receptor for Cr(III-ion that works in pure aqueous medium.
There are only few examples of receptors available in the literature, which binds exclusively to Cr (III) in an ensemble of several other competing metal ions. The Receptor 1 was reported by Li et. al, Chem. commun, 2008, 3387. It is a FRET based chemo sensor for detection of Cr(III) in ethanol/water (2/1, v/v) medium and it can be used as an imaging agent in HeLa cell.

Li et at also reported another Ferrocene Based Receptor 2 (Org. lett, 2008, 2557) for selective detection of Cr(III) in ethanol/water (1/1, v/v) with an association constant of 7.5×103 M−1. Receptors 3 (Anal. Methods, 2012, 3163; receptor is effective in acetonitrile/water (9/1, v/v)) and 4 (Anal. Methods, 2012, 2254; receptor is effective in methanol/water (9/1, v/v)) were developed by D. Das and his co-workers and both reagents showed high selectivity towards Cr(III) in predominantly organic medium. Receptor 5 is a BODIPY based Cr(III) sensors synthesised by D. Wang et al. (Tetrahedron Letters, 2010, 51, 2545). It can detect Cr(III) selectively in acetonitrite with binding stoichiometry of 2:2.

Further Hassan S S, et al. in Analytical Sciences (Impact Factor: 1.57). July 2005; 21(6):673-8 discloses use of a rhodamine-B chromate ion-associate complex as an electroactive material in a poly(vinyl chloride) membrane plasticized with o-nitrophenyloctyl ether as a solvent mediator. It is to be noted that the oxidation state for Chromium in chromate is (VI). They reported a Potentiometric Rhodamine B based Membrane Sensor for selective determination of Chromium ions [Cr(VI) & Cr(III)] in waste water. Firstly it can't detect Cr(III) directly. To detect Cr(III) they have adopted an indirect methodology in which Cr(III) was first oxidised to Cr(VI) by adding H2O2, that can be sensed by the reported sensor. Another important thing, this types of sensors cannot be applied for bio imaging application to monitor intra cellular Cr(III) activity.
In the present invention L1 it can selectively detect only Cr(III) in pure aqueous medium. It does not have any interferences of Cr(VI) at all. It is a fluoromertic as well as colorimetric sensor for Cr(III) detection. It can be used for monitoring intra cellular imaging of Cr(III). Fluorescence detection methods are more sensitive and simpler compare to other analytical methods.
Highly sensitive and selective fluorescence chemosensor for Cr3+ based on rhodamine B and a 4,13-diaza-18-crown 6-ether conjugate” is disclosed by Duliang Liu, in RSC Adv., 26 Nov. 2013, 4, 2563-2567, where detection of Cr3+ was possible only in predominantly non aqueous environment (3:2, MeOH—H2O (v/v).
There are only three previous reports that describe the use of reagents for the detection of Cr(III) in pure aqueous solution; (Mao, J et al. Org. Lett., 2007, 9, 4567-4570 and Mao, J et al. Anal. Bioanal. Chem., 2010, 396, 1197) one of them describes the interference by Fe3+. The most recent report reveals that a rhodamine derivative within a polymeric matrix could be utilized for specific detection of Cr (III) in pure aqueous medium and the hydrophobic micro-environment generated around the binding core of the receptor induces a favourable influence for the detection of Cr(III) (Macromol. Rapid Commun. 2014, 35, 323) However, the possibility of using these three molecular probes as an imaging reagent for studying the cellular uptake of Cr(III) is not explored and discussed.
While the receptors disclosed herein have the capability to detect Cr (III) in pure aqueous medium and in physiological condition. All other above examples suffer from a major drawback that they have this capability of detecting Cr (III) only in mixed aqueous organic solvents medium.
Detection of Cr (III) in pure aqueous solution is a very basic and vital need in the art, so that Cr(III) may be monitored for its presence and activity in cellular structure. There is also a need to detect Cr (III) in physiological fluids.